1. Field of the Invention
This invention relates to injection molding, and more particularly to the technique of injection molding for forming the central portion of a molded article of a core layer resin material and forming the surface portion thereof of a skin layer resin material and molding the article in the shape of sandwich in which the core layer resin material of the central portion is surrounded by the skin layer resin material.
2. Related Background Art
Lens barrels comprise a lens, a lens holding frame, a fixed cylinder, an intermediate cylinder, an operating ring, etc. and in which the lens holding frame is moved and controlled in a direction parallel to the optic axis by rotation of the operating ring about the optic axis through the intermediate cylinder. The intermediate cylinder can comprise a helicoid cylinder provided with helicoid portions on the inner and outer peripheries thereof to convert the rotational force of the operating ring about the optic axis into movement of the lens holding frame in a direction parallel to the optic axis. Adoption of a metal material such as aluminum (Al), brass (Bs) or the like as the material of the barrel has a physical advantage, but it poses numerous problems such as heaviness, the increased cost resulting from the machining of metal and the limit of improvement of productivity. Thus, injection molding using resin materials instead of metal materials for the lens barrel is practised. As suitable resin materials for the helicoid cylinder, there are known polycarbonate, ABS, polybutylene terephthalate (PBT), denatured polyphenylene oxide, etc., and further, resin materials having glass fiber or carbon fiber mixed therewith to reinforce the mechanical strength have been used.
The aforementioned polycarbonate having glass fiber mixed therewith can provide a helicoid cylinder of low cost, high accuracy, and mechanical strength, but still suffers from the problem that the glass fiber becomes exposed on the inner and outer peripheral surfaces of the helicoid cylinder; thus, improvement is required in the slidability of the helicoid cylinder when threadably engaged and sliding with inner and outer mating cylinders.
As a solution to the above-noted problem the cylinder member can be made into a layered structure. For example, by the use of the sandwich molding (hereinafter referred to as SW molding) as described in Japanese Patent Publication No. 28464/1975 (U.S. Pat. Nos. 3,809,519; 3,599,290; and 3,733,156), a resin material, having no glass fiber mixed therewith and excellent slidability and forming the surface layer of a helicoid cylinder, is first poured into the cavity of a metal mold for molding the helicoid cylinder, and then a resin material having glass fiber mixed therewith forming the core of the helicoid cylinder is poured into the cavity, whereby the surface layer is formed of smooth resin and the core is formed of resin of great mechanical strength, thereby obtaining a helicoid cylinder which can solve the above-noted problem.